1. Field of the Invention
This invention relates to a disk-type brushless motor, particularly to a disk-type brushless motor having one position sensor.
2. Description of the Prior Art
In recent years, there is a strong demand for brushless motors, suitable for such apparatus, especially disk-type brushless motors. Since such disk-type brushless motors are also useful as disk-type brushless fan motors for office machines, there is a demand, depending on machines to be incorporated therein, toward disk-type brushless fan motors which are extremely economical, small, thin and flattened.
As a brushless motor capable of satisfying the above-mentioned requirements to a highest possible degree, a disk-type brushless motor having an armature coil and a position sensor may be contemplated. Such a disk-type brushless motor cannot however be caused to rotate continuously, although it may be possible to rotate its magnet rotor over a certain predetermined range. Even if a motor equipped with only one armature coil and one position sensor should be able to rotate, it is unexpectable to obtain any large rotary force with such a single armature coil. For large rotary forces, it is indispensable to use two or more armature coils.
When designing, for example, a disk-type brushless motor having two armature coils as stator armatures, it has conventionally been necessary to use two position sensors. Namely, it has been required, for permitting continuous rotation, to design such a disk-type brushless motor into a two-phase disk-type brushless motor which requires two position sensors. Magnetoelectric transducers or converter elements such as Hall elements or Hall IC devices are often used as position sensors. These position sensors are however costly. It is certainly preferred from the viewpoint of mass production of economical, small and disk-type brushless motors, if each of such motors can be constructed with a single position sensor.
However, use of a single position sensor is accompanied by such a problem that similar to the above-mentioned motor with a single coil, the motor cannot start by itself when the position sensor detects the boundary area between an N pole and its matching S pole of the magnet rotor, namely, the dead point at the time of its start. Namely, the torque of a brushless motor reaches zero at the point where the current is switched over.
Japanese Utility Model Serial Nos. 56659/83 and 28958/83 disclose a disk-type brushless motor which includes a single position sensor element and two coils and which has a good efficiency and is inexpensive, wherein even with only one position sensor element, the self-start can be assured. According to these Utility Model Applications, it is possible to provide an inexpensive and useful disk-type brushless motor wherein a magnet rotor generates cogging in its most appropriate position and even with only one position sensor element, the magnet rotor can be constantly shifted from a dead point to a state in which it can self-start, thereby stopping the motor.
There has been proposed another disk-type brushless motor wherein only one armature coil is used to provide the same effect as in the use of two armature coils as described above.